The invention relates to a method for manufacturing extrudable products, wherein the material to be extruded is fed in a powder, pellet or granulate form, by using one or more supply means, into a processing cavity consisting of the volumes of the grooves in the rotor, of the grooves in the stator and of the clearance between them.
The invention further relates to an apparatus for manufacturing an extruded plastic product, the apparatus comprising at least one rotor and at least one stator, and a feed gap between them, grooves being provided on the other side of the feed gap for pressing the material to be extruded out of the apparatus when the rotor is rotated, the cross-section of the groove being substantially unchanging, and a countergroove being provided on the other side of the feed gap at least over a distance of the gap, the thread of the countergroove being opposite in direction to the groove provided on the other side of the feed gap.
The invention also relates to a plastic product containing matrix plastic to which 1 to 30% of barrier plastic has been added.
It is very difficult to process plastics having a high molecular weight with a conventional extruder consisting of a long screw and a cylinder. The yield of the extruder remains very small, since temperatures easily rise too high due to heat generated by the friction. Polymers that are difficult to process include for example fluoroplastics and polyethylenes having a great molar mass and a molecular weight of over 200 000 g/mol, in very hard plastics even over 300 000 g/mol. There are a number of similar materials that are hard to process and they have in common a high molecular weight and therefore a low melt flow rate, a high melting point and in some cases a narrow processing window, in other words for example the decomposition temperature of the plastic is close to its melting point.
Extrusion is particularly difficult at low temperatures when the extrusion temperature is kept only a few dozen degrees above the crystalline melting point, i.e. the temperature is typically about 30 to 40xc2x0 lower than normally in order that optional reactive components would not react too early. This situation is common especially when pipe grade chemically cross-linked pipes are produced. Problems occur since there are five different interconnected functions that are based on the action of the screw and that operate on the same axis and therefore at the same speed of rotation: feed, melting, mixing, homogenization and generation of pressure. In a conventional extruder, the screw is long and the ratio of the screw length to its diameter is most often about 20 to 30, and the screw comprises one, sometimes two or three, threads. The groove of the screw has been divided into different sections by varying the cross-section of the groove in the screw in such a way that the threads are often deep and positioned at long intervals from one another at the section where the supply takes place, and the cross-section is kept constant all the way in the melting zone but generally it is much bigger than at the outlet end. Thus, the material does not flow easily through the groove path and the end restriction causes a lot of frictional heat. In the mixing section the groove often has its own geometry, and in the end the groove becomes shallower and the generation of pressure begins. By means of this pressure a mass can be pressed even through a complicated tool. On the other hand, in a conventional extruder the total length of the groove around the screw is too long, in which case the ratio of the length of the groove to its cross-section becomes too great and it is therefore totally unsuitable for poorly flowable plastics.
U.S. Pat. No. 3,314,108 discloses an extruder comprising a conical rotor and conical stators provided in the exterior and interior of the rotor. The rotor comprises flat rectangular grooves for pressing the material to be extruded out of the extruder by rotating the rotor. However, with the aforementioned apparatus it is very difficult to process materials that are not easily workable. Also, the capacity will be limited.
EP 422,042 discloses an extruder comprising several conical stators and several conical rotors situated between them. The rotors and/or the stators are provided with calotte-shaped grooves for pressing the material to be extruded out of the extruder when the rotors are rotated. With this apparatus it is possible to manufacture very advantageously multilayer plastic pipes, but the processing of materials that are not easily workable creates a problem. Further, in the apparatus the moulding pressure is generated at the output end and the yield of the apparatus is therefore not entirely sufficiently good. Also, the material to be extruded is melted with heat that is provided from the outside, and it is therefore difficult to control the temperature, and the consumption of energy is relatively great.
U.S. Pat. No. 4,125,333 discloses an extruder comprising a long screw with threads, and a stator situated outside the screw and having same-handed threads. The same-handed threads of the stator produce a backflow, whereupon the material is mixed and the amount of heat generated by the friction easily increases to an uncontrolled level.
DE 2,558,238 also discloses an extruder comprising at its end a mixing section with either same-handed, opposite or direct grooves in the stator. Such an apparatus mixes the material very effectively, but it cannot be applied at all in equipment where the temperature of the material is to be adjusted accurately, since the amount of heat generated by the friction easily increases too much.
U.S. Pat. No. 3,712,783 discloses an extruder comprising a diverging feed zone. After the feed zone the material is pumped into a restriction zone. Thereafter the mass is pumped and caused to be extruded through an outlet. The structure of the apparatus is very complicated. The structure of the apparatus causes very high friction and because the extrusion pressure is raised near the outlet, the yield of the apparatus is very poor.
EP 0,678,069 discloses extruding multilayer pipes made of cross-linked polyethylene. The first step comprises extruding the centre layer of the pipe and thereafter the centre layer is coated with skins. The skins are used only for improving the flow characteristics of the pipe when the pipe is passing the heating tool. A special disadvantage of the apparatus is the need for a tool having spider legs, because the spider legs cause weld lines.
The purpose of the present invention is to provide a method and an apparatus with which it is relatively easy to also extrude poorly workable materials into a plastic product, and a plastic product with excellent properties.
The method according to the invention is characterized in that the cross-sectional area of said cavity decreases at least partially continuously along the axis of the extruder, and by the relative rotational movement of the stator and the rotor said material is forced to proceed along an x-axis, whereby the frictional heat caused by the shear melts a part of the material forming a bed consisting of mainly unmelted particles and some melt around them, enabling the formation of a processing cavity that is completely filled at a certain cross-section at a distance from the end of the apparatus, end hence a rise in the pressure to a level higher than needed for pushing the material through a die later on along the x-axis.
Also, the apparatus according to the invention is characterized in that after the feed section there is a shear zone, the countergroove being positioned substantially along the entire length of the shear zone, and that the cross-sectional area of the processing cavity, consisting of the volumes of the grooves in the rotor, of the grooves in the stator and of the clearance between them, decreases at least partially continuously along the axis of the extruder.
Further, the product according to the invention is characterized in that the barrier plastic is positioned in the product in such a way that it forms a laminar structure.
The essential idea of the invention is that the material is extruded in a processing cavity that consists of the grooves of the rotor and the stator and of the clearance provided between them, and that the volume of this cavity decreases at least partly in the axial direction, so that the material is forced in the axial direction to a smaller cross-sectional space, whereupon the heat generated by the friction resulting from the shearing melts the material at least partly, which results in an increase in the pressure already a distance before the end of the apparatus. Further, it is essential in the invention that plastic can be processed to such a small extent that at the orifice of the extruder, if there is no heating means, the mass flow contains particles that have not melted completely. For example in polyethylene such particles are visible as lighter particles in the transparent mass. According to tests that have been conducted, the unmelted particles do not impair the properties of the product in any way. Most preferably, at least a part of the flight of the countergroove matches every flight of the grooves provided on the other side of the feed gap, in which case the countergroove is narrower than the flights between the grooves provided on the other side of the feed gap. The idea of a preferred embodiment is that the width of the countergroove is about 30 to 50% smaller than the width of the groove provided on the other side of the feed gap. The idea of another preferred embodiment is that the gradient of the countergroove is about half of the gradient of the groove provided on the other side of the feed gap. The idea of a third preferred embodiment is that the size of the countergroove or the volume of the processing cavity varies in such a way that the ratio of the cross-sectional area of the initial part of the groove to the cross-sectional area of the final part thereof approximately equals the ratio of the density of the solid mass to the bulk density of the mass to be extruded. The idea of a fourth preferred embodiment is that the feed gap is annular and decreases evenly in diameter, i.e. it has a conical form, at least over a distance of its length, so that the diameter of the feed gap at the end where material is fed is greater than its diameter closer to the material outlet end, and that the ratio of the diameter of the wider section of the cone to the diameter of the narrower section approximately equals the ratio of the density of the solid mass of the material to be extruded to the bulk density of the mass. The idea of a fifth preferred embodiment is that the groove supplying material has a substantially triangular or semicircular cross-section. Further, the ratio of the length of the extrusion screw to the greatest diameter of the screw is preferably equal to or less than about 10, for example. Most preferably the ratio for a cylindrical extruder is between 3 and 6. Also, in a prior art extruder the ratio of the groove length of the screw to the cross-sectional area of the groove is over 20 1/mm, whereas in the apparatus according to the invention, the ratio of the length of the groove to the cross-sectional area of the groove is less than 20 1/mm.
The invention has the advantage that the melting and homogenization of the mass require as little deformation energy as possible. When the pressure is raised sufficiently high at a relatively early stage, the yield of the apparatus can be improved substantially. It is possible to determine for an extruder a theoretical yield indicating the amount of ideal material the apparatus produces when the rotor rotates one round. In a prior art apparatus, the actual yield is rather low with respect to the theoretical maximum, typically about 10 to 15%. With the present apparatus, it possible to achieve as high a yield as 50% from the theoretical maximum yield by means of the countergrooves and the decreasing cross-sectional surface of the cavity. The unchanging cross-section of the melt-conveying groove does not force the plastic away from the area of the groove even in the final section of the groove. When a countergroove is provided on the other side of the feed gap, the flight between the supply grooves can be made wide since the countergroove grips the material to be extruded, which therefore travels effectively forward in the apparatus, i.e. the amount of wasted energy decreases essentially. When the flights of the countergrooves are positioned in such a way that they match the flights of the grooves on the other side of the feed gap, backflow is prevented effectively and the material is simultaneously made to rotate advantageously. With an apparatus provided with the screw geometry according to the invention, masses that would otherwise be difficult to process can also be extruded well. It is possible to process, for example, cross-linkable polyethylene having a molecular weight of over 200 000 g/mol and, in very tough plastics, as high as over 300 000 g/mol. Even with such materials the yield of the apparatus can be easily maintained for example over 100 kg/h, whereas the yield of a conventional piston (ram) extruder is typically about 25 kg/h, for example. When the countergroove or the processing volume or the feed gap is made to change in a ratio equalling the ratio of the density of the solid mass of the material to be extruded to the bulk density of the mass, air can be prevented from passing with the mass flow. Also, overfeeding will not result in such great overpressures that in conventional machines will lead to a breakdown or at least a runout of the torque. The flow of material in a substantially triangular or a semicircular groove is easy, temperature gradients are smaller and the rotating motion of the material within the groove cross-section is very effective. With a conical screw it is possible to shorten the total length of the groove of the screw and therefore to reduce the amount of heat generated by the friction caused by the flow. Further, the surface of a conical screw is typically about 50% greater than the surface of a screw of a conventional cylindrical extruder having an equal yield. This is very advantageous for mastering the need for cooling or heating. When round single-screw apparatuses are equipped with countergrooves, the extruder needs cooling in the feed zone because of excessive pressure and friction. The important improvement in the present apparatus is that no extra cooling is needed and the process is very stable and controllable. Nor is any extra cooling (cooling fans etc.) needed in the machine barrel despite the very high outputs that are achieved.